Prosecutors say they don’t believe a jury likely would convict Dennis Watters Jr. of murder. Instead, jurors could side with Watters, who claims he was defending himself when he shot Ryan Mumm, 20.

Witnesses told detectives Mumm was armed with a handgun and fired the weapon before he and Watters, 41, engaged in a shootout. A bullet struck Mumm in the head. He died July 15 at an Everett hospital.

“At this point, we don’t believe we could disprove self-defense beyond a reasonable doubt and that’s what the law requires,” Snohomish County deputy prosecutor Laura Twitchell said.

That doesn’t mean Watters is escaping criminal prosecution for his part in the July 14 gun battle.

On Friday, Twitchell charged him with first-degree assault with a firearm. Watters may have been defending himself when he shot Mumm, but he broke the law when he chased after a car and fired additional shots, according to prosecutors.

“The defendant joined a fight in which he had no direct involvement and brought a gun,” Twitchell wrote. “Then he shot and killed Ryan Mumm. After shooting Mumm, the defendant chased the car Mumm was in; rammed it; and continued shooting at it.”

If convicted of the assault charge, Watters faces up to 15 years in prison. A second-degree murder conviction would carry a 15- to 23-year prison sentence.

Snohomish County sheriff’s detectives have spent the last month piecing together the events that led up to the violence.

Witnesses told investigators that the catalyst may have been a dispute over a $20 marijuana deal earlier that day.

Mumm was with a friend who was offered a small amount of marijuana by a group of people outside the Arlington Safeway. The sale went sideways when Mumm’s friend drove off with the dope without paying, court papers said.

The dealers gave chase but Mumm and his friend got away.

The two groups crossed paths later and reportedly exchanged blows.

But the fistfight didn’t put an end to the dispute.

The two groups eventually agreed to meet up that night at Blue Stilly to settle their beef. Both sides called for backup, recruiting friends. One girl reportedly called her father, complaining that she’d been assaulted by Mumm’s friend.

That man reportedly called Watters to help him confront the guys who hit his daughter, court papers said. The man reportedly confirmed that Watters would be carrying a gun.

Mumm and his friend drove to the park, along with three of their friends, who were in a different car. No one was at the park and Mumm’s friend began to drive out of the park. That’s when a Honda Accord collided with the man’s BMW. A witness told police that’s when Mumm got out of the BMW and pointed a gun at the Honda driver. A witness said Mumm fired the gun in the air.

Meanwhile, Watters arrived in his pickup. Detectives were told that after Mumm fired his gun, a shootout happened between him and Watters. A witness also told police that he saw Watters ram his pickup into the BMW on Highway 530 and shoot at the vehicle.

Mumm’s friend eventually parked at a gas station. That’s where sheriff’s deputies found the injured man. Mumm was motionless in the passenger seat with an obvious gunshot wound to the head. He was rushed to the hospital, but died the next day.

Detectives later reviewed surveillance video from the gas station and saw the slain man’s friend hide something. The man eventually told police he’d hidden Mumm’s gun.

Investigators spoke with Watters, who reportedly admitted that he shot Mumm, court papers said. He told detectives that his pickup came face-to-face with a BMW and he saw the passenger point a gun at him through the windshield. He said he held up both hands while the BMW backed up. He said the passenger held a gun out the window and fired it into the air. He said as the car drove around his pickup, the passenger shot at him. He said that’s when he grabbed his gun, loaded it and returned fire. He said he fired three rounds and believed he’d struck the passenger in the shoulder.

He reportedly admitted that he chased after the BMW and tried to bump it off the road, court papers said. Police arrested Watters four days after the shooting. On Friday he remained behind bars on $1 million bail.

“Although we could not save Ryan, he will be saving the lives of many people through his deep and generous act of love by organ donation. We will miss him forever and find comfort in the knowledge that we will be with him again one day,” they wrote in the statement.